General Assist Technical Reference
Outdoor Fan Control
AOTA72LALT and AOTA90LALT
How the outdoor fan speed is controlled in cooling, dry and heating operation
The outdoor fan does not operate at one fixed speed. The control board selects one of 16 fan steps according to the operating mode, refrigerant-system demand and outdoor temperature. This allows the system to maintain suitable refrigerant pressures while using only the airflow required at the outdoor heat exchanger.
1. Why the outdoor fan speed changes
Cooling and dry operation
In cooling mode, the outdoor coil operates as the condenser. It must reject the heat removed from indoors, together with the heat produced by the compressor.
- High outdoor temperature or high cooling demand: more outdoor airflow is required, so the fan is driven at a higher step.
- Lower outdoor temperature or light cooling demand: less airflow is required, so the fan slows down or cycles intermittently.
Running the outdoor fan too fast in cool outdoor conditions can reduce condensing pressure excessively. The controller therefore reduces fan airflow to keep the refrigerant system within its intended pressure and temperature range.
Heating operation
In heating mode, the outdoor coil operates as the evaporator and absorbs heat from the outdoor air. Fan airflow is adjusted to support the required evaporation pressure and heating capacity.
- Higher heating demand: the system generally requires more heat transfer through the outdoor coil, so a higher fan step may be selected.
- Reduced demand or conditions requiring pressure control: the fan may slow or operate intermittently.
- Cold or humid weather: frost can form on the outdoor coil. Fan operation may change or stop during defrost control; this is separate from the normal fan-step table.
Key point: Outdoor temperature is an important input, but it is not the only factor. The final fan step is selected by the unit's control logic using operating mode and current refrigerant-system requirements. Fan speed can therefore change even when the outdoor temperature appears stable.
2. Outdoor fan operating sequence
| Condition | Control behaviour |
|---|---|
| Compressor starts | The compressor and outdoor fan normally start together. |
| Initial fan run | The fan operates at approximately 500 rpm for 20 seconds after start-up. When the initial commanded speed is 300 rpm or lower, it starts at 300 rpm for 20 seconds instead. |
| Normal operation | The controller moves to the required fan step according to mode, outdoor temperature and system demand. |
| Compressor stops | The outdoor fan continues for approximately 60 seconds after the compressor stops. |
| Fan restart protection | After the fan stops, it remains off for at least 5 seconds before restarting. |
3. Fan-step speed table
Steps 7 to 16 are continuous fan speeds. Steps 1 to 6 use intermittent operation at 300 rpm.
| Fan step | Cooling / Dry | Heating | ||
|---|---|---|---|---|
| AOTA72LALT | AOTA90LALT | AOTA72LALT | AOTA90LALT | |
| 16 | 850 rpm | 860 rpm | 880 rpm | 880 rpm |
| 15 | 850 rpm | 860 rpm | 840 rpm | 840 rpm |
| 14 | 820 rpm | 830 rpm | 810 rpm | 810 rpm |
| 13 | 730 rpm | 730 rpm | 730 rpm | 730 rpm |
| 12 | 620 rpm | 620 rpm | 600 rpm | 600 rpm |
| 11 | 500 rpm | 500 rpm | 500 rpm | 500 rpm |
| 10 | 420 rpm | |||
| 9 | 360 rpm | |||
| 8 | 320 rpm | |||
| 7 | 300 rpm continuous | |||
| 6 to 1 | 300 rpm intermittent operation - see below | |||
4. Intermittent fan control
When less airflow is required than the continuous 300 rpm available at fan step 7, the controller does not attempt to run the fan below its stable minimum speed. Instead, it cycles the fan between:
- 300 rpm for 7 seconds, followed by
- 0 rpm for a step-dependent off time.
| Fan step | Mode | Fan off time | Fan run time |
|---|---|---|---|
| 6 | Intermittent 1 | 6 sec | 7 sec at 300 rpm |
| 5 | Intermittent 2 | 12 sec | |
| 4 | Intermittent 3 | 19 sec | |
| 3 | Intermittent 4 | 26 sec | |
| 2 | Intermittent 5 | 33 sec | |
| 1 | Intermittent 6 | 40 sec |
How to interpret this: Step 6 provides the greatest average airflow of the intermittent steps because the fan is off for only 6 seconds. Step 1 provides the least average airflow because each 7-second run is followed by 40 seconds off.
When the controller changes from a normal continuous step to an intermittent step, the first cycle starts with a 7-second run at 300 rpm. If the required intermittent step changes while cycling, the controller completes the current timed portion before moving to the new interval.
5. What is normal during diagnosis
| Observed behaviour | Interpretation |
|---|---|
| Fan starts at a different speed before settling | Normal initial 20-second fan-control sequence. |
| Fan continues after compressor stops | Normal 60-second fan overrun. |
| Fan repeatedly runs and stops | May be normal intermittent control at steps 1 to 6, particularly when only a small amount of outdoor-coil airflow is required. |
| Fan speed changes without a thermostat adjustment | Normal modulation as outdoor temperature and refrigerant-system demand change. |
| Fan stops during heating in cold weather | May be associated with defrost or other system control. Check the complete operating state before diagnosing a fan fault. |
Do not diagnose the fan from speed alone. Confirm the commanded operating mode, compressor status, outdoor temperature, refrigerant pressures, thermistor readings, defrost status and any active fault code. A fan that cycles at 300 rpm may be operating exactly as commanded.
6. Summary
- The outdoor fan has 16 commanded steps.
- Steps 7 to 16 use continuous speeds from 300 rpm to a maximum of 880 rpm.
- Steps 1 to 6 use timed 300 rpm intermittent operation.
- In cooling, fan control maintains suitable heat rejection and condensing pressure.
- In heating, fan control supports outdoor-coil heat absorption and evaporation-pressure control.
- Outdoor temperature influences the selected step, but the controller also responds to system demand and refrigerant conditions.
Applicable models: AOTA72LALT and AOTA90LALT. Technical information should be used together with the relevant service manual and live system measurements.